1. Field of the Invention
This invention relates to the field of hydraulically-actuated friction clutches and brakes, particularly to those used in automatic transmissions for automotive vehicles.
2. Description of the Prior Art
Hydraulically-actuated friction clutches and brakes used in automatic transmissions frequently include balance dams that provide accurate, consistent control of gear ratio changes. Clutches and brakes respond to the magnitude application of pressurized hydraulic fluid in a cylinder, in which an actuating piston moves due to pressure forces to engage the device and due to a spring force to disengage it.
Balance dams are replacing ball check valves, which are very dependent on angular disposition, ball-to-wall clearance, and flow rate in order to achieve accurate and consist gear ratio changes. A balance dam creates a centrifugal hydraulic pressure that opposes and balances centrifugal hydraulic pressure developed on the apply side of the piston. These centrifugal pressures are caused by rotation of the entire friction element. In this way, pressure effects, due to the speed of rotation, can be eliminated or reduced to a point where the control system pressure alone creates the force operative to engage the friction element. When the cylinder is vented, a compression spring returns the piston to the disengaged position.
In both the ballcheck and balance dam designs, a spring force is required to return the apply-piston to the disengaged position. In the balance dam design, the return spring force and balance dam centrifugal force act in the same direction to return the apply-piston to the disengaged position and to hold it there.